Replacement Gate Process for Semiconductor Devices

ABSTRACT

Disclosed is a method of forming a semiconductor device. The method includes providing a precursor having a substrate and gate stacks over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer. The method further includes depositing a dielectric layer over the substrate and the gate stacks and filling spaces between the gate stacks; and performing a first chemical mechanical planarization (CMP) process to partially remove the dielectric layer. The method further includes performing an etching process to remove the second HM layer and to partially remove the dielectric layer, thereby exposing the first HM layer. The method further includes performing a second CMP process to at least partially remove the first HM layer.

PRIORITY

This claims the benefits of U.S. Prov. App. Ser. No. 62/272,272, entitled “Replacement Gate Process for Semiconductor Devices,” filed Dec. 29, 2015, herein incorporated by reference in its entirety.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advancements to be realized, similar developments in IC processing and manufacturing are needed.

One advancement implemented in some IC designs has been the replacement of the typically polysilicon gate electrode with a metal gate electrode to improve device performance with the decreased feature sizes. One process of forming a metal gate electrode is termed a replacement-gate or “gate-last” process in which the metal gate electrode is fabricated “last,” by replacing a polysilicon gate. This allows for reduced number of subsequent processes, including high temperature processing, that is performed after the formation of the final gate. However, there are challenges to implementing such IC fabrication processes, especially with scaled down IC features in advanced process nodes, such as 20 nanometer (nm), 16 nm, and beyond. For example, different areas of an IC may have different gate lengths and/or undergo different fabrication steps between the formation of the gates and the replacement of the gates with metal gates. It is challenging to maintain a uniform height among the polysilicon gates in different areas of an IC. The variations in the polysilicon gates' heights present issues for subsequent replacement gate processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a flow chart of a method of forming a semiconductor device, according to various aspects of the present disclosure.

FIGS. 2 and 3 are embodiments of some steps of the method in FIG. 1, in accordance with some embodiments.

FIGS. 4, 5, 6, 7, 8, 9, 10, 13, and 14 illustrate cross-sectional views of forming a target semiconductor device according to the method of FIG. 1, in accordance with an embodiment.

FIGS. 11 and 12 illustrate top views of forming a target semiconductor device according to the method of FIG. 1, in accordance with an embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Referring to FIG. 1, shown therein is a method 10 of forming a semiconductor device according to various aspects of the present disclosure. As will be discussed below, the device will undergo a replacement-gate process where typical polysilicon gates are replaced with final (metal) gates. One goal of some embodiments of the method 10 is to provide the gates with a near uniform height right before they are replaced, thereby eliminating or reducing issues commonly seen with typical “gate-last” processes, such as within-die gate height loading issues, dielectric residue issues, and incomplete poly-cut issues. The method 10 is an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after the method 10, and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. The method 10 is described below in conjunction with FIGS. 4-14 which are cross-sectional views and top views of a device 100 according to various aspects of the present disclosure.

As will be shown, the device 100 may be an intermediate device fabricated during processing of an IC, or a portion thereof, that may comprise static random access memory (SRAM) and/or other logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as p-type field effect transistors (PFET), n-type FET (NFET), metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof. Further, the transistors may be multi-gate transistors, such as FinFETs.

At operation 12, the method 10 (FIG. 1) provides a precursor of the device 100. For the convenience of discussion, the precursor is also referred to as the device 100. Referring to FIG. 4, the device 100 includes a substrate 102 and gate stacks 104 disposed over the substrate 102. The substrate 102 is a silicon substrate in the present embodiment. Alternatively, the substrate 102 may comprise another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GalnAs, GaInP, and/or GaInAsP; or combinations thereof. In yet another alternative, the substrate 102 is a semiconductor on insulator (SOI). Even though not shown, the substrate 102 may include active regions separated by isolation features. For example, the active regions may be n-type doped active regions and/or p-type doped active regions, and may be planar active regions and/or non-planar active regions (e.g., fins). The isolation features may be shallow trench isolation (STI) features, field oxide, LOCal Oxidation of Silicon (LOCOS), and/or other suitable structures. Furthermore, the substrate 102 may include epitaxial features.

In the embodiment shown, each of the gate stacks 104 includes an electrode layer 106, a first hard mask (HM) layer 108 over the electrode layer 106, and a second HM layer 110 over the first HM layer 108. In an embodiment, the electrode layer 106 includes polysilicon. The electrode layer 106 may be formed by suitable deposition processes such as low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced CVD (PECVD). In an embodiment, the first HM layer 108 includes a dielectric material comprising a nitride, such as silicon nitride or silicon oxynitride; and the second HM layer 110 includes a dielectric material comprising an oxide, such as silicon oxide. For example, a silicon nitride HM layer 108 may be formed by CVD using chemicals including Hexachlorodisilane (HCD or Si2Cl6), Dichlorosilane (DCS or SiH2Cl2), Bis(TertiaryButylAmino) Silane (BTBAS or C8H22N2Si) and Disilane (DS or Si2H6). For example, a silicon oxide HM layer 110 may be formed by thermal oxidation. In various embodiments, the first HM layer 108 and the second HM layer 110 comprise different dielectric materials. Each of the first and second HM layers, 108 and 110, may be formed by a suitable deposition method such as chemical oxidation, thermal oxidation, atomic layer deposition (ALD), chemical vapor deposition (CVD), and/or other suitable methods. The gate stacks 104 may include other layer(s). For example, the gate stacks 104 may include an interfacial layer between the electrode layer 106 and the substrate 102.

In embodiments, the various layers of the gate stacks 104 may be first deposited as blanket target layers over the substrate 102, and then patterned using a process that includes one or more photolithography processes and one or more etching processes. A typical photolithography process includes coating a resist layer over the target layers, soft baking the resist layer, and exposing the resist layer to a radiation using a mask (or photomask). The photolithography process further includes post-exposure baking, developing, and hard baking thereby removing portions of the resist layer and leaving a patterned resist layer as the masking element. The masking element provides various opening through which the target layers are etched using a dry etching, a wet etching, or other suitable etching methods. For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. For example, a wet etching process may comprise etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; a solution containing hydrofluoric acid (HF), nitric acid (HNO₃), and/or acetic acid (CH₃COOH); or other suitable wet etchant. In an embodiment, a patterned resist is used as a masking element for etching the first and second HM layers, 110 and 108. Subsequently, the first and second HM layers, 110 and 108, are used as a masking element for etching the electrode layer 106. The one or more etching processes remove portions of the target layers, resulting in the gate stacks 104 standing over the substrate 102.

Although not shown, the device 100 may further include gate spacers on sidewalls of the gate stacks 104. For example, the gate spacers may include a dielectric material, such as silicon oxide, silicon nitride, or silicon oxynitride, and may be formed by one or more deposition and etch-back techniques.

The gate stacks 104 may be formed in different areas or regions of the substrate 102. In the embodiment shown, the gate stacks 104 are formed in substrate regions 102 a, 102 b, 102 c, and 102 d. The number of substrate regions and the number of gate stacks in each region as shown in FIGS. 4-14 are for simplification and ease of understanding and do not necessarily limit the embodiments to any types of devices, any number of devices, any number of regions, or any configuration of structures or regions. The different substrate regions 102 a-d may be used for forming different types of devices, such as SRAM devices and logic devices. The gate stacks 104 in different substrate regions may have different pattern pitches and/or different pattern widths along the “x” direction. For example as shown in FIG. 4, the gate stacks 104 in the substrate region 102 b have a narrower pattern width and a smaller pattern pitch than those in the substrate region 102 c. In an embodiment, the substrate region 102 b is for forming SRAM cells, while the substrate region 102 c is for forming input/output (IO) cells.

Still referring to FIG. 4, in some embodiments, the gate stacks 104 may have different heights along the “z” direction. Particularly, the height (or thickness) of the HM layer 110 may vary among the substrate regions. For example, the gate stacks 104 in the substrate region 102 a has a height H₁ that is less than a height H₂ of the gate stacks 104 in the substrate region 102 b, which in turn is less than a height H₃ of the gate stacks 104 in the substrate region 102 c. Again, the heights H₁, H₂, and H₃ in FIG. 4 are for illustrative purposes. The differences in the gate heights may result from a variety of factors. For example, when the HM layers 110 and 108 are used as a masking element for etching the electrode layer 106, etchants may be distributed unevenly across the surface of the device 100 due to variations in pattern densities, pattern widths, and/or pattern pitches. As a result, the HM layer 110 may be consumed relatively more in some regions (e.g., region 102 a) and relatively less in some other regions (e.g., region 102 d). For another example, some substrate regions may undergo more fabrication processes than other substrate regions after the gate stacks 104 have been formed. For example, some substrate regions may undergo epitaxial growth processes in the source/drain regions thereof and the epitaxial growth processes may be for n-type epitaxy or for p-type epitaxy. The HM layer 110 may be consumed unevenly in these additional fabrication processes.

In a replacement-gate process, the HM layers 110 and 108 need to be removed in order to expose the electrode layer 106 for replacement. However, the different gate heights present a challenge for typical replacement-gate processes. For example, in a typical replacement-gate process, the electrode layer 106 may be overly etched in the substrate region 102 a, and/or the HM layer 108 may be incompletely etched in the substrate region 102 d, leaving dielectric residues over the top of the electrode layer 106. Both the over-etching of the electrode layer 106 and the under-etching of the HM layer 108 are problematic for subsequent replacement processes. An object of the present disclosure is to completely remove the HM layers 110 and 108 and to provide the electrode layer 106 with a uniform height for the ease of subsequent replacement processes.

Referring to FIG. 1, at operation 14, the method 10 forms an etch stop layer (ESL) 112 over the substrate 102 and the gate stacks 104. Referring to FIG. 5, the ESL 112 is formed over top and sidewall surfaces of the gate stacks 104 and the top surface of the substrate 102. In an embodiment, the ESL 112 is formed to have a conformal cross-sectional profile (in the “x-z” plane). In an alternative embodiment, the ESL 112 does not have a conformal cross-sectional profile. In an embodiment, the ESL 112 comprises a nitride that may be the same as or different from the nitride of the first HM layer 108. The ESL 112 may be formed by CVD, PECVD, ALD, or other suitable deposition techniques. The ESL 112 is optional in some embodiments, i.e., the method 10 may skip operation 14 and proceeds from operation 12 to operation 16.

At operation 16, the method 10 (FIG. 1) deposits an inter-layer dielectric (ILD) layer 114 over the substrate 102, the gate stacks 104, and the ESL 112. Referring to FIG. 6, the ILD layer 114 covers the various structures including the gate stacks 104 and the ESL 112, and fills the spaces between the various structures over the substrate 102. In an embodiment, the ILD layer 114 includes an oxide such as tetraethylorthosilicate oxide, un-doped silicate glass, doped silicon oxide such as borophosphosilicate glass, fused silica glass, phosphosilicate glass, boron doped silicon glass, or other suitable oxide materials. The ILD layer 114 may be deposited by a PECVD process, a flowable CVD (FCVD) process, or other suitable deposition technique. For example, the FCVD process may include depositing a flowable material (such as a liquid compound) on the substrate 102 to fill the spaces between the various structures and converting the flowable material to a solid material by a suitable technique, such as annealing. The ILD layer 114 may or may not have a flat top surface as deposited.

At operation 18, the method 10 (FIG. 1) performs a first chemical mechanical planarization (CMP) process 116 to partially remove the ILD layer 114. Referring to FIG. 7, in the present embodiment, the CMP process 116 partially removes the ILD layer 114 until a top surface of the ESL 112 is exposed, thereby planarizing a top surface of the device 100. In an embodiment, the ESL 112 is used as an end point for the CMP process 116. For embodiments where the ESL 112 is not included in the device 100, the second HM layer 110 may be used as an end point for the CMP process 116. For example, the CMP process 116 may apply CMP slurry over the ILD layer 114 and then grind and buff the device 100 until the end point (the ESL 112) is detected in situ. In some embodiments, the CMP slurry may be suspended in a mild etchant, such as potassium or ammonium hydroxide. The CMP slurry may include ferric nitrate, peroxide, potassium iodate, ammonia, silica, alumina, and/or other slurry materials as applicable. In some embodiments, the CMP slurry also includes organic additives configured to provide a better topography after the CMP process 116.

At operation 20, the method 10 (FIG. 1) etches the ILD layer 114, the ESL 112, and the second HM layer 110 to expose the first HM layer 108. In an embodiment, operation 20 includes a first etching 118 (operation 26) followed by a second etching 120 (operation 28), as illustrated in FIGS. 2, 8, and 9.

Referring to FIGS. 2 and 8 collectively, the first etching 118 (operation 26) has no etch selectivity or low etch selectivity with respect to the materials of the ILD layer 114, the ESL 112, and the second HM layer 110. In another word, the first etching 118 etches (or removes) the ILD layer 114, the ESL 112, and the second HM layer 110 at about the same rate. As a result, the top surface of the device 100 remains about flat during the first etching 118. In the present embodiment, the duration of the first etching 118 is controlled by a timer, which may be set based upon the thickness of the second HM layer 110 and the ESL 112 and the etching rate of the same such that the first etching 118 stops right before the first HM layer 108 is exposed. As illustrated in FIG. 8, a thin layer of the second HM layer 110 remains on the first HM layer 108 in an embodiment. In another embodiment, the first etching 118 completely removes the second HM layer 110 and may also slightly etch the first HM layer 108. Comparing the device 100 in FIGS. 7 and 8, even though the gate stacks 104 have different heights in different regions 102 a-d before the first etching 118 is performed (FIG. 7), they now have near uniform heights after the first etching 118 is completed (FIG. 8).

Referring to FIGS. 2 and 9 collectively, after the first etching 118 is completed, the device 100 undergoes the second etching 120 (operation 28). The second etching 120 uses a different etchant than that of the first etching 118. The second etching 120 is highly selective to the ILD layer 114 and the second HM layer 110. In another word, the second etching 120 removes the ILD layer 114 and the second HM layer 110 at a much higher rate than it removes the ESL 112 and the first HM layer 108. As a result, the second HM layer 110 is completely removed, the ILD layer 114 is partially removed, and the ESL 112 and the first HM layer 108 remain substantially unchanged (although some small amount of the ESL 112 and the first HM layer 108 may also be removed). In an embodiment, the duration of the second etching 120 is controlled by end-point detection. The end-point detection detects an element, such as nitrogen, that is included in the first HM layer 108 but not in the second HM layer 110 and the ILD layer 114. When the second etching 120 is completed, the first HM layer 108 and portions of the ESL 112 are exposed, as shown in FIG. 9. FIG. 9 shows that the top surface of the device 100 is uneven, with the ESL 112 and the first HM layer 108 being higher than the ILD 114. FIG. 9 further shows that the ESL 112 is higher than the first HM layer 108 in this embodiment. In various embodiments, the first HM layer 108 may be higher or lower than the ESL 112 or they may be at about same level, depending on the etch selectivity of the two layers in the second etching 120.

In an embodiment, both the first etching 118 and the second etching 120 are dry etching processes. The dry etching processes may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., Cl₂, CHCl₃, CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. In a further embodiment, both the first etching 118 and the second etching 120 are performed in the same dry etching process chamber. For example, the process chamber is supplied with a first etching gas (or gas mixture) that has no etch selectivity or low etch selectivity with respect to the materials of the ILD layer 114, the ESL 112, and the second HM layer 108. The first etching 118 is performed with the first etching gas for the duration discussed above. Then, the first etching gas is switched to a second etching gas (or gas mixture) that is highly selective to the materials of the ILD layer 114 and the second HM layer 108. The second etching 120 is performed with the second etching gas for the duration discussed above.

In an embodiment, the method 10 (FIG. 1) may perform an annealing process to enhance the quality of the ILD layer 114 subsequent to operation 20.

At operation 22, the method 10 (FIG. 1) performs a second CMP process 122 to at least partially remove the first HM layer 108. Referring to FIG. 10, in the present embodiment, the CMP process 122 completely removes the first HM layer 108, thereby exposing the electrode layer 106. The CMP process 122 also partially removes the ESL 112, thereby planarizing a top surface of the device 100. In an embodiment, the CMP process 122 applies CMP slurry that is selective to the materials of the ESL 112 and the first HM layer 108. In another word, the ESL 112 and the first HM layer 108 are removed at a higher rate than the ILD layer 114 during the buffing or polishing operations of the CMP process 122. In an embodiment, a cleaning process may be subsequently performed to remove any CMP residue. For example, the cleaning process may use deionized water (DIW).

Referring to FIG. 10, after operations 18, 20, and 22, the device 100 is provided with a planar top surface, and the electrode layer 106 is provided with a near uniform height without over-etching and without any residue from the HM layers 108/110. This lays a good foundation for subsequent gate replacement processes.

At operation 24, the method 10 (FIG. 1) performs further processes to fabricate a final IC device. In the present embodiment, operation 24 includes one or more steps that replace the electrode layer 106 with final gate stacks. One embodiment of operation 24 is shown in FIG. 3 that includes operation 30 followed by operation 32. At operation 30, the method 10 cuts at least some of the electrode layer 106 into multiple electrode segments. This is referred to as a “cut poly” step. At operation 32, the method 10 replaces the electrodes and electrode segments with final gate stacks. Operations 30 and 32 are briefly discussed below in conjunction with FIGS. 11-14.

FIG. 11 shows a top view (in the “x-y” plane) of the device 100 post operation 22. Referring to FIG. 11, the electrode layer 106 includes a plurality of elongated shapes (electrodes 106) that are oriented widthwise in the “x” direction and lengthwise in the “y” direction. Each of the electrodes 106 is surrounded by the ESL 112 on its sidewall surfaces. The “cut poly” step of operation 30 may include photolithography and etching processes. For example, the photolithography process forms a masking element (e.g., a patterned resist) that covers portions of the electrodes 106 and exposes the remaining portions of the same. The etching process etches the exposed portions of the electrodes 106 through the masking element, thereby cutting them into segments. FIG. 12 illustrates a result of the “cut poly” step. Referring to FIG. 12, each of the electrodes 106 in the regions 102 a and 102 b is cut into two segments 106′. The two segments 106′ are separated by a gap 124. In an embodiment, the gaps 124 are subsequently filled with an isolation material to electrically isolate the segments. In embodiments, the electrodes 106 in any regions 102 a-d may be cut into two or more segments. In advanced process nodes such as 16 nm or smaller, the “cut poly” step helps achieve higher device density by forming regular patterns using one photomask and cutting the regular patterns using another photomask. The electrodes 106 prepared according to embodiments of the present disclosure are particularly suitable for the “cut poly” step because they have near uniform heights and are free of oxide or nitride residue at their top surface (FIG. 10). Electrodes prepared according to traditional methods may have oxide or nitride residues at their top surface. Consequently, the electrodes may not be completely cut and isolated during the “cut poly” step.

At operation 32, the electrodes 106 and the electrode segments 106′ (collectively “the electrodes 106”) are replaced with final gate stacks. This may involve one or more etching and deposition processes. For example, the one or more etching processes remove the electrodes 106 to form openings 126 (FIG. 13), and the one or more deposition processes form final gate stacks 128 in the openings 126 (FIG. 14). In one example, the final gate stacks 128 include an interfacial layer, a gate dielectric layer, a work function metal layer, and a metal fill layer. The interfacial layer may include a dielectric material such as silicon oxide or silicon oxynitride, and may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable dielectric. The gate dielectric layer may include a high-k dielectric material such as hafnium oxide, zirconium oxide, lanthanum oxide, titanium oxide, yttrium oxide, strontium titanate, other suitable metal-oxides, or combinations thereof. The gate dielectric layer may be formed by ALD and/or other suitable methods. The work function metal layer may be a p-type or an n-type work function layer. The p-type work function layer comprises a metal with a sufficiently large effective work function, such as titanium nitride, tantalum nitride, ruthenium, molybdenum, tungsten, platinum, or combinations thereof. The n-type work function layer comprises a metal with sufficiently low effective work function such as titanium, aluminum, tantalum carbide, tantalum carbide nitride, tantalum silicon nitride, or combinations thereof. The work function metal layer may include a plurality of layers and may be deposited by CVD, PVD, and/or other suitable process. The metal fill layer may include aluminum, tungsten, cobalt, copper, and/or other suitable materials. The metal fill layer may be formed by CVD, PVD, plating, and/or other suitable processes.

Operation 24 may include further steps, such as forming source/drain/gate contacts and forming metal interconnects, to complete the fabrication of the device 100.

Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and the formation thereof. An embodiment of the present disclosure uses a process including a CMP process, a dry etching process, and another CMP process to completely remove hard mask layers atop a poly layer prior to gate replacement. Such process results in near uniform height in the poly layer without over-etching of the poly layer. This provides a good foundation for subsequent gate replacement. Embodiments of the present disclosure can be easily integrated with existing manufacturing processes in the advanced process nodes.

In one exemplary aspect, the present disclosure is directed to a method of forming a semiconductor device. The method includes providing a precursor having a substrate and gate stacks over the substrate. Each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer. The method further includes depositing a dielectric layer over the substrate and the gate stacks and filling spaces between the gate stacks. The method further includes performing a first chemical mechanical planarization (CMP) process to partially remove the dielectric layer; and performing an etching process to remove the second HM layer and to partially remove the dielectric layer, thereby exposing the first HM layer. The method further includes performing a second CMP process to at least partially remove the first HM layer.

In another exemplary aspect, the present disclosure is directed to a method of forming a semiconductor device. The method includes providing a precursor having a substrate and gate stacks disposed over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer. The method further includes forming an etch stop layer (ESL) over the substrate and covering top and sidewalls of the gate stacks; and depositing an inter-layer dielectric (ILD) layer over the ESL and filling spaces between the gate stacks. The method further includes performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer; performing a dry etching process to partially remove the ESL, the ILD layer, and the second HM layer, thereby exposing the first HM layer; and performing a second CMP process to at least partially remove the first HM layer.

In another exemplary aspect, the present disclosure is directed to a method of forming a semiconductor device. The method includes providing a precursor having a substrate and gate stacks disposed over the substrate. Each of the gate stacks includes a poly layer, a nitride hard mask (HM) layer over the poly layer, and an oxide HM layer over the nitride HM layer. The method further includes forming an etch stop layer (ESL) over top and sidewalls of the gate stacks. The ESL comprises a nitride. The method further includes depositing an inter-layer dielectric (ILD) layer covering the ESL and the gate stacks and filling spaces between the gate stacks. The ILD layer comprises an oxide. The method further includes performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer until the ESL is exposed; and performing a first etching process to partially remove at least the ESL, the ILD layer, and the oxide HM layer. After the performing of the first etching process, the method further includes performing a second etching process selectively tuned to etch the ILD layer and the oxide HM layer while the nitride HM layer remains substantially unchanged, thereby exposing the nitride HM layer. The method further includes performing a second CMP process to selectively remove the nitride HM layer while the ILD layer remains substantially unchanged.

The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of forming a semiconductor device, comprising: providing a precursor having a substrate and gate stacks over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer; depositing a dielectric layer over the substrate and the gate stacks and filling spaces between the gate stacks; performing a first chemical mechanical planarization (CMP) process to partially remove the dielectric layer; performing an etching process to remove the second HM layer and to partially remove the dielectric layer, thereby exposing the first HM layer; and performing a second CMP process to at least partially remove the first HM layer.
 2. The method of claim 1, wherein: the electrode layer comprises polysilicon; the first HM layer comprises a nitride; the second HM layer comprises an oxide; and the dielectric layer comprises another oxide.
 3. The method of claim 1, further comprising: before the depositing of the dielectric layer, forming an etch stop layer (ESL) over top and sidewalls of the gate stacks, wherein the ESL is used as an end point for the first CMP process.
 4. The method of claim 3, wherein the etching process includes a first etching followed by a second etching, wherein: the first etching has a low etch selectivity with respect to materials of the dielectric layer, the second HM layer, and the ESL; and the second etching selectively removes the dielectric layer and the second HM layer while the first HM layer remains substantially unchanged in the second etching.
 5. The method of claim 4, wherein both the first etching and the second etching are dry etching.
 6. The method of claim 4, wherein the first etching and the second etching are performed in the same process chamber.
 7. The method of claim 4, wherein the first etching is controlled by a timer and the second etching is controlled by detecting an element contained in the first HM layer but not in the second HM layer and the dielectric layer.
 8. The method of claim 1, wherein the second CMP process applies slurry selectively tuned for removing the first HM layer while the dielectric layer remains substantially unchanged.
 9. The method of claim 1, wherein the gate stacks have different heights.
 10. A method of forming a semiconductor device, comprising: providing a precursor having a substrate and gate stacks disposed over the substrate, wherein each of the gate stacks includes an electrode layer, a first hard mask (HM) layer over the electrode layer, and a second HM layer over the first HM layer; depositing an inter-layer dielectric (ILD) layer over the gate stacks and filling spaces between the gate stacks; performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer; performing a dry etching process to partially remove, the ILD layer and the second HM layer, thereby exposing the first HM layer; and performing a second CMP process to at least partially remove the first HM layer.
 11. The method of claim 10, wherein: the first HM layer comprises a nitride; the second HM layer comprises an oxide; and the ILD layer comprises another oxide.
 12. The method of claim 10, wherein the dry etching process includes a first etching followed by a second etching, wherein: the first etching removes the ILD layer and the second HM layer at about the same etching rate; and the second etching removes the ILD layer and the second HM layer at a higher etching rate than it removes the first HM layer.
 13. The method of claim 12, wherein the first etching is switched to the second etching before the first HM layer is exposed.
 14. The method of claim 12, wherein the first etching and the second etching are performed in the same process chamber.
 15. The method of claim 10, wherein the second CMP process applies slurry that removes the first HM layer at a higher rate than it removes the ILD layer.
 16. The method of claim 10, further comprising: cutting an electrode layer in one of the gate stacks into multiple segments.
 17. A method of forming a semiconductor device, comprising: providing a precursor having a substrate and gate stacks disposed over the substrate, wherein each of the gate stacks includes a poly layer, a nitride hard mask (HM) layer over the poly layer, and an oxide HM layer over the nitride HM layer; forming an etch stop layer (ESL) over top and sidewalls of the gate stacks, the ESL comprising a nitride; depositing an inter-layer dielectric (ILD) layer covering the ESL and the gate stacks and filling spaces between the gate stacks, the ILD layer comprising an oxide; performing a first chemical mechanical planarization (CMP) process to partially remove the ILD layer until the ESL is exposed; performing a first etching process to partially remove at least the ESL, the ILD layer, and the oxide HM layer; after the performing of the first etching process, performing a second etching process selectively tuned to etch the ILD layer and the oxide HM layer while the nitride HM layer remains substantially unchanged, thereby exposing the nitride HM layer; and performing a second CMP process to selectively remove the nitride HM layer while the ILD layer remains substantially unchanged.
 18. The method of claim 17, wherein the first and second etching processes are dry etching processes and are performed in the same process chamber.
 19. The method of claim 17, wherein the first etching process has low etch selectivity with respect to the ESL, the ILD layer, and the oxide HM layer.
 20. The method of claim 17, further comprising: cutting a poly layer in one of the gate stacks into multiple poly segments. 